The present invention relates to a projection aligner and an exposure method used for fine pattern formation of solid devices such as semiconductor devices, magnetic bubble devices and superconducting devices.
Heretofore, fine patterns such as wiring of solid devices including LSIs have been formed by using the projection exposure method, especially the reduction projection exposure method. In accordance with the above described method, an image of the mask pattern is focused and transferred onto a substrate with resist applied thereon by using a projection lens. Since the resolution limit in the reduction projection exposure method is in proportion to the exposure wavelength and is in inverse proportion to the numerical aperture of the protection lens, resolution improvement has been promoted by shortening the exposure wavelength and increasing the numerical aperture of the projection lens. On the other hand, the depth of focus of the projection lens is in proportion to the exposure length, and is in inverse proportion to the square of the numerical aperture of the projection lens. By attempting to improve the resolution, therefore, the depth of focus has been abruptly decreased. That is to say, it is difficult to make fine patterns compatible with insurance of sufficient depth of focus. Especially when high resolution is aimed at, the depth of focus becomes very shallow. Projection exposure methods are discussed in Chapter 4, pp. 87 to 93 of a book entitled "Semiconductor Lithography Technique", written by Koichiro Ho, and published by Sangyo Tosho, for example.
While circuit patterns are made finer with the advance of higher integration of LSIs, device functions arranged hitherto in two dimensions begin to be arranged in three dimensions. As a result, large topographies are caused on surfaces of LSIs. In addition, focal planes of mask patterns themselves are curved (field curvature), and substrates themselves are declined as a whole. Therefore, it is not possible to make the focal plane coincide with the substrate surface. In addition, the disagreement between the focal plane and the substrate surface is increased.
On the other hand, the depth of focus of projection lenses is being decreased with the improvement in resolution as described before. Therefore, it has become difficult to confine the above described device surface having a large topography over the entire face of the exposure field within the above described depth of focus and resolve fine patterns.
At the same time, one cannot help giving up the idea of making device dimension fine in order to ensure the depth of focus required for fabrication of an LSI.
In order to solve the above described problems, it is attempted to smooth the substrate surface by using the multilayer resist method or the like, reduce the field curvature and the substrate inclination, and improve the focus latitude from the viewpoint of process by using the CEL (Contrast Enhancement Lithography) method. However, the above described decrease in depth of focus of the projection lens is not sufficiently solved by these methods.
The multilayer resist method is described in Journal of Vacuum Science and Technology, B-1 (4), pp. 1235 to 1240 (1983), for example.
The present inventors paid their attention to the fact that the depth of focus of a lens can be made effectively deep by using the so-called focus latitude enhancement exposure (FLEX) method in which a plurality of images focused onto different positions on an identical optical axis are superposed each other. As a result, the present inventors thought out one technique for solving the above described problems. This technique was already applied for a patent (U.S. patent application Ser. No. 083,211, filed on Aug. 10, 1987). The above described our preceding patent application discloses that a photoresist layer is exposed under the condition that focal planes of a mask pattern (i.e., positions whereat accurately focused images of the mask pattern are formed) are set at a plurality of positions on the optical axis which are separated each other. As a more concrete technique of the above described exposure in which focal planes are set at a plurality of positions, it is disclosed in an embodiment that a stage carrying a substrate is moved in the direction of the optical axis for the fixed focal plane. When the stage is moved, however, high precision is demanded for the position control of the stage. Therefore, there is a fear that the resolution is lowered because the precision in stage position is limited.